One form of advertising sign comprises a rectangular frame with a flexible sheet of material stretched across the frame. The advertisement is on the sheet which is usually backlighted. The sheet must be tensioned to remove all wrinkles, wrinkles detracting from the appearance of the sign. Desirably, the sign is also able to compensate for dimensional changes in the sheet if the sheet is heated up. This is a particular problem in outdoor signs where the temperature of the sign changes considerably depending on whether or not it is being subjected to direct sunlight.